A wireless communication eliminates burden of wiring operations in a traditional wired communication and further serves for a utilization as a technology for realizing a mobile communication. For example, as a regular standard with regard to a wireless LAN (Local Area Network), IEEE (The Institute of Electrical and Electronics Engineers) 802.11 can be exemplified. IEEE802.11a/g has been already widely spread.
According to the standard of IEEE802.11a/g, in a 2.4 GHz band or 5 GHz band frequency, a modulation method for achieving a communication speed of 54 Mbps at maximum (physical layer data rate) is supported by utilizing an orthogonal frequency division multiplexing (Orthogonal Frequency Division Multiplexing: OFDM). Also, in IEEE802.11n that is an extended standard thereof, a further higher bit rate is realized by adopting an MIMO (Multi-Input Multi-Output) communication system. Herein, the MIMO refers to a communication system provided with a plurality of antenna elements on both a transmitter side and a receiver side for realizing spatially multiplexed streams (widely known). Although a high throughput (High Throughput: HT) above 100 Mbps can be achieved by IEEE802.11n, a realization of a further higher speed is demanded along with an increase in the information amount of transmission contents.
For example, since the number of antennas in the MIMO communication device is increased and the number of streams to be spatially multiplexed is increased, it is possible to improve the throughput in a one-to-one communication while the backward compatibility is maintained. However, in future, an improvement in the throughput for the plurality of users as a whole is demanded in addition to the throughput per user in the communication.
The working group for IEEE802.11ac aims to establish a wireless LAN standard in which a frequency band smaller than or equal to 6 GHz is used and a data transmission speed exceeds 1 Gbps, and for the realization, like multi user MIMO (MU-MIMO) or SDMA (Space Division Multiple Access), a space division multiple access system where a wireless resource on a spatial axis is shared by a plurality of users is potent.
At present, the space division multiple access is under review as one of fundamental technologies for a next generation mobile phone series system based on a time division multiple access (Time Division Multiple Access: TDMA) such as PHS (Personal Handyphone System) or LTE (Long Term Evolution). Also, in a wireless LAN field, a one-to-many communication is being paid attention as described above, but an application example is rarely met. This is probably because it is difficult to efficiently multiplex the plurality of users in the packet communication.
Incidentally, a communication system is proposed in which two technologies of the carrier sense in the conventional IEEE802.11 and the space division multiple access by an adaptive array antenna are combined with each other by using the RTS, CTS, and ACK packets composed of a packet format that maintains a backward compatibility with the conventional IEEE802.11 (for example, see PTL 1).
Herein, in a case where the space division multiple access is applied to the wireless LAN, a case of multiplexing variable length frames on a same time axis is conceivable. No problem occurs if transmission data lengths with respect to each of a plurality of users are all the same size, but if frame lengths to be multiplexed vary from each other due to a difference in the transmission data lengths, the total transmission power abruptly changes along with an increase or decrease in the number of the multiplexed frames during a transmission period. If the frames having the different lengths are multiplexed and transmitted without change, the reception power abruptly changes along with the increase or decrease in the number of multiplexed frames on the reception side, which triggers an unstable operation in terms of an auto gain control (Auto Gain Control: AGC), and also, a possibility exists in that problems occur from various viewpoints such as an instability of power distribution within the frame with regard to RCPI (Received Channel Power Indicator) regulated by IEEE802.11. For this reason, even if the transmission data length for each user varies, the frames multiplexed on the same time need to be transmitted while eventually having the same frame length.
For example, in a system of a fixed frame format such as a conventional cellular system, it is possible to carry out a compensation of frames or the like through an insertion of data for diversity (for example, see PTL 2), a scheduling of allocated times (for example, see PTL 3), a variable data rate (for example, see PTLs 4 and 5), and a variable channel configuration (for example, see PTL 6). In contrast to this, because the structure is fundamentally different from a system adopting a variable frame format such as the wireless LAN, it is difficult to apply these conventional technologies.